Java 9 인터페이스의 개인 메소드

interface my_int{
   public abstract void multiply_vals(int a, int b);
   public default void add_vals(int a, int b){
      sub_vals(a, b);
      System.out.print("Default method result ");
      System.out.println(a + b);
   }
   private void sub_vals(int a, int b){
      System.out.print("Private method result ");
      System.out.println(a - b);
   }
   private static void div(int a, int b){
      System.out.print(" Private static method result ");
      System.out.println(a / b);
   }
}
public class my_new_int implements my_int{
   @Override
   public void multiply_vals(int a, int b){
      System.out.print("Abstract method result ");
      System.out.println(a * b);
   }
   public static void main(String[] args){
      my_int in = new my_new_int();
      in.multiply_vals(11, 34);
      in.add_vals(78, 0);
   }
}

Abstract method result 374
Private method result 78
Default method result 78